The present invention relates to a rotor and flexible drive shaft assembly for a progressive cavity pump or motor. The invention is particularly concerned with a progressive cavity pump or motor having an outer member, usually a stator, which has female helical gear formation thereon having n starts and a rotor, rotatable within said stator and having a male helical gear formation of the same pitch but with n.+-.1 starts.
In former years the rotor was driven by means of a drive shaft having a universal joint at each end. More recently, however, the rotor is driven by a flexible drive shaft see, for example, GB-A-1220848; GB-A-1285377; GB-A1562277 and GB-A-2244517.
Such a drive arrangement has been very satisfactory. However, the method of connecting the drive shaft to the rotor is somewhat complex and expensive and is not always entirely satisfactory if it is necessary to rotate the drive shaft in either one rotational direction or the other.